Project Summary Hypertrophic cardiomyopathy (HCM), as the leading cause of sudden cardiac death in young adults, affects approximately 1 in 500 individuals and is characterized by diastolic dysfunction, fibrotic tissue formation, impaired contractile properties and, most evidently, left ventricle hypertrophy and increased myofilament Ca2+ sensitivity. To date, there are no effective non-invasive therapies for HCM. Mutations in sarcomeric proteins?including cardiac troponin C (cTnC), the Ca2+ sensor in contraction?seem to be the causative agents in the disease. The disease mechanisms and how the various mutations lead to myopathy, however, are poorly understood. More specifically, the Ala8Val (A8V) mutation in cTnC has been identified among HCM patients and within three independent probands and led us to develop the knock-in mouse model based on its pathogenicity. The A8V mutation in humans and mice causes left ventricular hypertrophy and hypercontractility, atrial enlargement, and increased myofilament Ca2+ sensitivity (the latter only tested in mice). Recently, cardiac myosin light chain kinase (cMLCK) knock-out mice were shown to exhibit reduced ejection fraction. This is attributed to the reduction in regulatory myosin light chain phosphorylation, which is known to decrease myofilament Ca2+ sensitivity. Consequently, we hypothesize that a conditional cMLCK knock-out of a cTnC-A8V (HCM phenotype) mouse can normalize cardiac function toward wild-type. We believe this will improve heart morphology, hemodynamics, and protein expression levels, as measured via ECHO, pressure-volume loops, histopathology, heart to tibia length ratios, cardiomyocyte Ca2+ and twitch transients, and western blots for Ca2+ handling proteins. In addition, we will use advanced bioinformatics tools such as proteomics and RNA sequencing to quantify overall changes in these mouse models. Cardiac myosin light chain kinase has thus far been characterized as a dedicated kinase, making it a promising therapeutic target. As such, the proposed work can set the stage for novel, targeted heart disease therapy for HCM.